In the stereo vision system of a video camera, in order to obtain the depth or parallax information of an object in the scene, it is necessary to obtain projection relation between the coordinates of a certain point of the object in the world coordinate system and the coordinates of an imaging point of the object on the imaging plane. The video camera calibration technology is developed to record such information.
This technology is designed to obtain the internal and external parameters of the video camera. Geometrical and optical features of the video camera are called internal parameters, and the position relation of the coordinate system of the video camera relative to the space coordinate system is called external parameters.
Generally, three methods are available for calibrating the video camera, including traditional calibration, active vision-based calibration, and auto-calibration.
Traditional calibration refers to: using a precision machined calibration block as the reference template to calculate the internal and external parameters of the video camera by establishing mapping between the existing points on the three-dimensional (3D) coordinates on the calibration block and the image points thereof.
Active vision-based calibration refers to: controlling the video camera to make special movement, for example, rotating around the optical center or moving horizontally, and calculating the internal parameters based on the specialty of such movement.
Auto-calibration refers to: assuming that no internal parameter is changed and mapping between image points is established when shooting different images, calibrating the video camera according to such mapping between the image points and special constraints between the imaging points in multiple images.
Through research and practice, the inventor has identified the following problems with the art mentioned above.
1. Regarding the traditional calibration, a calibration block is needed each time when the angle or position of the video camera is changed. However, placing the calibration block in a scene for a long time may make the scene to be inharmonious, and is not allowed in some scenes. Moreover, it is also inconvenient to replace the calibration block each time when the angle or position of the video camera is changed.
2. Regarding active vision-based calibration, the video camera needs to be controlled to make special movement. Such movement, however, cannot be perceived or controlled in some circumstances. In such cases, this method cannot be used.
3. Regarding auto-calibration, the video camera is calibrated on the assumption that the internal parameters remain unchanged and mapping between image points is determined. Thus, the calibration is neither precise nor stable. Therefore, such method is not applicable if any internal parameter is changed.